Thin-film hard disk magnetic media are widely used in read/write memory devices in computers. Increasingly, there is an effort in the thin-film medium industry to achieve higher recording density (Futamoto et al.). Among the magnetic properties which are important to a high recording density are:
(1) Coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density.
(2) Bit shift or peak shift, a phenomenon which refers to the broadening between voltage peaks, as well as reduction in peak amplitude, which occurs in the read voltage waveform, where the peak-to-peak broadening time is typically less than about 25 nsec. It is desired to achieve low bit shifting, inasmuch as bit shifting limits the resolution at which adjacent peaks can be read, and thus places an upper limit on recording density.
(3) Signal-to-noise is the relative strength of background noise to the data signal obtained during reading of the thin-film medium. The higher the ratio of signal to noise, the cleaner the sensed information signal and the less likelihood there is of having erroneous readings.
(4) Bit density is the density in the medium that bits are recorded. A bit is identified as a flux transition in the medium, and bit density is measured the number of flux transitions per unit length. Typically, the higher the bit density, the lower the signal-to-noise ratio.
(5) Pulse width is a parameter that is generally inversely related to the coercivity. That is, the higher the coercivity, the narrower a bit needs to be sensed.
(6) Signal amplitude, or peak-to-peak amplitude of a single pulse, as a function of recording frequency. The recording density of the medium is related to the drop in signal amplitude at increasing recording frequency.
(7) Signal resolution, defined as the ratio of the high-frequency track average amplitude divided by the low-frequency track amplitude. The recording frequency at which 70% resolution is achieved represents one measure of information storage density on the disk.
Thin-film media or disks are commonly prepared by sputtering a thin magnetic film on a substrate, such as a textured, plated aluminum substrate. The disk is typically prepared by sputtering an underlayer, such as a chromium underlayer, onto the substrate surface, then sputtering a cobalt-based magnetic thin film over the underlayer. A protective, lubricating carbon overcoat may be applied over the thin-film layer by sputtering.
A variety of magnetic film alloys have been reported in thin-film media of the type just described (e.g., Yogi, Miller, Sanders, Shiroishi). U.S. Pat. No. 4,888,514 discloses a thin film disk having a cobalt-nickel layer sputtered over a chromium underlayer. A coercivity of 650 Oe (Oersteds), a saturation magnetization of greater than 10,000 Gauss, and a loop squareness ratio of greater than 0.9 were reported. Magnetic thin-film media with chromium underlayer with cobalt-nickel or cobalt-nickel-chromium alloy magnetic layers are also disclosed in U.S. Pat. Nos. 4,833,044, 4,816,127, and 4,735,840.
U.S. Pat. No. 4,786,564 issued to Chen et al. also discloses the use of nickel-phosphorus (Ni/P) sublayers on an aluminum substrate. This process was developed to control the nucleation and growth of the crystalline structure of the magnetic media and also to prevent nonuniformities in the substrate surface from affecting the magnetic characteristics of the media.
Longitudinal magnetic recording media for hard disk drives have typically employed a textured Ni/P-coated Al substrate. The advent of lower fly heights and higher recording densities have placed severe requirements on the substrate. Ni/P-coated Al substrates of thicknesses below 35 mils have shown significant problems in maintaining good fly characteristics below 4 microinches. As the thickness of the substrate is reduced, these substrates have also shown a greater susceptibility to handling damage and mechanical flatness.
Nonmetallic substrates, such as canasite.TM. (glass-ceramic), glass or carbon substrates, have smoother surfaces and higher flexural strength. As a result, they are capable of providing superior fly properties and are potential replacements for Ni/P-coated Al substrates.
A canasite.TM. substrate conventionally is placed in a high vacuum deposition system and preheated to 200.degree.-300.degree. C. DC magnetron sputtering is then used to sequentially deposit a Cr underlayer followed by the magnetic alloy and a thin protective carbon overcoat.
Longitudinal magnetic recording media deposited on canasite.TM. substrates have exhibited lower coercivities and squarenesses than similar media deposited on standard aluminum substrates. Media on canasite.TM. substrates exhibit lower outputs, higher noise and increased bit-shifts over comparable media deposited on Ni/P-coated Al substrates. They exhibit lower coercivities and squareness, and higher oxygen levels. More specifically, the coercivity is usually lower by 175-460 Oe, HF signal amplitude is typically 25% lower, and bit shift is greater by 4.5 ns.
Increased coercivity in a medium formed on a glass substrate has been reported. In the approach reported in this reference, a glass substrate is coated with a sputtered NiP coating to a thickness of about 2,000 .ANG.. After texturing the coated substrate, the substrate is successively sputtered with chromium, to a thickness of about 500 .ANG. layer, and then with a Co-based magnetic alloy, to a thickness of about 600 .ANG.. The medium showed an enhanced coercivity which was nonetheless significantly lower than that achieved in a comparable medium formed on a conventional Ni/P-coated Al substrate.
Co-owned U.S. patent application for "Thin-Film Recording Medium," Ser. No. 790,585, pending, filed Nov. 8, 1991, discloses a thin-film magnetic recording medium composed of a glass or ceramic substrate. Formed on the substrate is a sputtered Ni/P sublayer having a thickness between about 80-1,000 .ANG., preferably 100-500 .ANG., a sputtered chromium underlayer having a thickness between 1,000-3,000 .ANG., preferably at least about 2,000 .ANG., and a sputtered magnetic layer having a thickness between about 200-800 .ANG.. The medium has a substantially higher coercivity, HF signal amplitude, and signal-to-noise ratio than the same medium formed in the absence of the sputtered nickel-phosphorus sublayer.
Co-owned U.S. patent application for "Thin-Film Recording Medium with Thin Chromium Underlayer," Ser. No. 964,745, pending, filed Oct. 22, 1992 discloses a magnetic thin-film medium in which the chromium underlayer has a thickness between about 100-300 .ANG., and a sputtered magnetic thin-film layer formed from a Co alloy. The medium is characterized by high coercivity and signal-to-noise ratio.